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The short answer? Buy a quarter teaspoon, fill it up, and use it on your face twice per day.
The long answer? The math and logic are approximate for the quarter teaspoon estimation, and depend on your face and neck size, as well as the formulation:
1.) Measure your face.
When a sunscreen is developed, cosmetic scientists apply it to a screen at a density of 2.0 mg/cm2 to measure SPF rating, a measure of how long you can stay in the sun without erythema (skin redness) and burning. If you apply over this amount, you won’t get any additional UV protection. But if you apply under this amount (which most persons do), you are getting significantly less UV protection than you think.
As our former staff writer John Su once measured in an old blog post on sunscreen, his face length and width were 12 inches and 9 inches, respectively. Through a series of steps, he calculated that he needed 1.2439 mL of sunscreen for his face. Since a quarter teaspoon is 1.2323 mL, that’s just about perfect!
Buying a separate 1/4 teaspoon to measure your daily facial application amount of sunscreen is approximately the amount that most people need to apply to achieve 2.0 mg/cm2.
2.) Most people apply less than half of the sunscreen they need.
Formulations undergo rigorous testing to achieve SPF that involves very thick application of product — 2 mg/cm2 of skin, or about half a 8 oz. bottle to cover the average body. It covers a grid until it appears almost white, and light is reflected onto t
vhe grid to measure absorbance capacity to determine the overall effectiveness of the product. This is how the SPF rating is achieved for all sunscreens.
Unfortunately, the average person only applies 1/3 to 1/2 of the amount of sunscreen needed to achieve the SPF rating on the bottle. So your SPF 30 sunscreen is more like an SPF 10-15 with the average application.
It’s even worse with powder formulations. The average consumer only applies 1/14 of the amount of sunscreen needed to achieve the SPF rating on the bottle, so your SPF 15 powder formula is more like an SPF 1 formula:
3.) SPF 20 + SPF 50 = SPF 50.
With perfect application, sunscreens do not have an additive effect. Instead, think of sunscreens more as having a limiting effect — they limit the number of rays that get through, but you can’t increase above the limit of the highest number.
For instance, if you apply a very thick application of an SPF 50 formula, it will allow 1/50 UVB rays through, or about 2% (source).
If you apply a very thick application of an SPF 20 formula, it will allow 1/20 UVB rays through, or about 5%.
If you apply both formulations perfectly at the same time, the SPF 20 formula allows 5% of rays through. The SPF 50 formula will block some of those, but 2% of the overall rays will still get through. This is regardless of which formulation you apply first.
If you want optimal UV protection for your face, get a quarter teaspoon and use it!